Full breathing masks that include an inner face mask and an outer face mask are known and have been used for many years in environments where the inhalation of an ambient atmosphere is unsuitable or impossible, for instance when the wearer is situated under water, when the partial pressure of oxygen is insufficient, for instance at high altitudes, and fails to administer the amount of oxygen required, or the atmosphere contains or is feared to contain poisonous or harmful substances, such as carbon monoxide for instance.
Full breathing masks of this kind comprise an outer face mask which includes a hood-like intermediate wall which extends out from the inside of the mask and which has a free end. The intermediate wall has a concave or a convex side, where the concave side is intended to embrace and lie against the wearer's chin. The perimeter of the inner face mask abuts the convex side of the intermediate wall and the wearer's face outwardly thereof. The inner face mask and the intermediate wall enclose or surround the wearer's mouth and nose. The inner face mask which partially abuts the intermediate wall and partially the wearer's face defines a first chamber with the wearer's face, i.e., his or her mouth and nose. The outer face mask defines a second chamber together with the wearer's face and the outside of the inner face mask.
Such a full breathing mask is worn on the wearer's face when in use, wherewith the outer face mask and the inner face mask are held pressed against the wearer's face with the aid of straps on the outer face mask. The breathing mask is connected by a hose or the like to a source of breathing gas, typically a container in which the gas is contained under high pressure.
The gas taken from the pressurized container is reduced by a first pressure regulator to a pressure on the order of 7 bars and prior to being delivered to the breathing mask is reduced typically to a pressure in the order of 2.5 millibars or 25 mm water column by a second pressure regulator.
The gas from the second pressure regulator is delivered first to the outer face mask and then to the inner face mask through openings therein. The openings in the inner face mask include check valves for preventing exhalation gas from flowing to the outer face mask.
The gas present in the inner face mask is inhaled by the wearer who, by exhalation, then delivers the exhalation gas to the inner face mask, this gas containing a high concentration of carbon dioxide. The exhaled gas is forced through a passageway to the ambient surroundings, via the inner face mask. The passageway includes a check valve which prevents the ambient atmosphere from entering the inner face mask.
When a fresh breath is taken, the exhalation gas that is present in the airways of the wearer and in the inner face mask will be inhaled first and thereafter fresh breathing gas is taken from the outer face mask. The amount of gas which subsequent to exhalation is inhaled is designated dead space. The volume defined by the breathing path of the wearer is designated anatomic (inner) dead space whereas the volume of re-inhaled gas being outside the breathing path is designated dynamic outer dead space.
One drawback with known breathing masks is that the gas of exhalation is pressed between the intermediate wall and the inner face mask and out into the outer face mask. The fresh breathing gas present in the outer face mask then becomes contaminated with carbon dioxide from the exhaled gas. Thus, the dynamic outer dead space does not solely consist of the volume of gas present in the inner face mask, but also in the volume of gas present in the outer face mask.
FIGS. 1 and 2 illustrate one known so-called full breathing mask comprising an outer face mask 1 and an inner face mask 11. The full breathing mask includes a wall 21 which is situated distal from a wearer when the mask is donned and which is common to both the outer face mask 1 and the inner face mask 11. The wall 21 includes a speech membrane 22.
The outer delimiting part of the outer face mask 1 extends from the common wall 21 towards the wearer and merges with an essentially circular or oval perimeter 3, 4, which is intended to abut the wearer's face and to enclose a facial region that includes the eyes, nose, mouth and chin of the wearer. The upper part of the perimeter is referenced 3 and the lower part thereof is referenced 4. The outer limitation of the outer face mask 1 also includes a visor 8 and, although not shown, an opening that is provided with a breathing valve for the supply of breathing gas from a gas source, for instance from a container carried by the mask wearer. The breathing mask is held in place by straps 2 the ends of which are fastened in the outer face mask of the full breathing mask as shown in FIG. 1.
The upper part of the perimeter 3 of the outer face mask 1 has a generally concave face-abutment surface which merges with a part shown at the bottom of the figure, this part forming an intermediate wall 4 that extends outward from the inside of the outer face mask 1. The intermediate wall 4 extends from the inside of the outer face mask 1 and in over the wearer's chin and terminates in a free edge 5 below the wearer's mouth. The intermediate wall 4 is bowl-shaped and embraces the chin of the wearer and therewith has a concave abutment surface 7 which faces towards the wearer's chin and has on the other side a convex surface 6 which is turned away from the wearer's face.
The inner face mask 11 spreads from the common wall 21 toward the wearer's face wherewith the perimeter 12, 13 of the wall is in direct abutment or indirect abutment with the wearer's face. An upper perimeter part 12 directly abuts the wearer's face above the upper edge 5 of the intermediate wall 4 of the outer face mask 1, and then passes to a lower perimeter part 13 which lies against the convex surface 6 of the intermediate wall 4. The perimeter part 12 of the inner face mask 11 has a convex abutment surface.
The wall 21 common to both the outer face mask 1 and the inner face mask 11 includes an opening 14 which connects the interior of the inner face mask 11 with the surroundings. The opening 14 is provided with a check valve 15 which allows gas to pass from the interior of the inner face mask 11 to the surroundings, while preventing the ingress of ambient atmosphere into the inner face mask 11.
The inner face mask 11 has at least one opening 16 which connects the inner face mask 11 with the outer face mask 1. The opening 16 is provided with a check valve 17 which allows gas to pass from the outer face mask 1 to the inner face mask 11 but prevents the flow of gas in the opposite direction.
When the breathing mask is in use, breathing gas that has a pressure of about 25 mm water column is delivered to the wearer as he or she inhales, wherewith gas flows into the outer face mask 1 and then through the opening 16 and into the inner face mask 11 and from there into the airways of the wearer. As the wearer then breaths out, the exhalation gas is pressed into the inner face mask 11 and from there through the outlet opening 14 to the surrounding atmosphere. This exhalation gas contains carbon dioxide produced in the wearer's lungs in an amount on the order of 5%. With the next breath taken by the wearer, the exhalation gas present in the inner face mask 11 will be inhaled before fresh breathing gas reaches the upper airways of the wearer. For this reason breathing masks are produced with an inner face mask that has the smallest possible volume in practice. This volume is designated dynamic outer dead space.
It has been found that ideal flow of exhalation gas is not achieved with such known breathing masks. It has also been found that exhalation gas having an elevated carbon dioxide content leaks from the inner face mask 11 to the outer face mask 1 during exhalation. This leakage probably takes place in the region of the intermediate wall 4 against which the lower perimeter 13 of the inner face mask 11 abuts. Since the inner face mask 11 has a higher pressure than 25 mm water column during the exhalation phase, the pressure in the outer face mask 1 is lower than the pressure in the inner face mask 11. The force with which the lower perimeter 13 of the inner face mask 11 lies against the intermediate wall 4 is not sufficient to prevent exhalation gas from flowing between the intermediate wall 4 and the lower perimeter of the inner face mask 11. As a result, the clean breathing gas in the outer face mask 1 becomes contaminated with carbon dioxide. This leakage also results in the volume of the outer face mask 1 being contaminated with carbon dioxide. Consequently it is not only the carbon-dioxide-containing gas from the upper airways and the inner face mask 11 that reaches the lungs of the wearer before fresh breathing gas is received, but also the volume of carbon-dioxide-containing gas present in the outer face mask 1 that is inhaled prior to the delivery of fresh breathing gas. As a result of this larger amount of carbon dioxide that is first inhaled, the rated minute ventilation will be greater and more fresh breathing gas will be consumed. This drawback and others not explicitly described have been overcome with a full breathing mask according to the present disclosure.